Crystal structure of cis-2-(2-carboxycyclopropyl)glycine (CCG-III) monohydrate

The title compound is an example of a ‘false conglomerate’ with two molecules of opposite handedness in the asymmetric unit. Each molecule exists as a zwitterion, with proton transfer from the amino acid carboxylic acid group to the amine group. In the crystal, the components are linked by N—H⋯O and O—H⋯O hydrogen bonds, generating (100) sheets.

The title compound, C 6 H 9 NO 4 ÁH 2 O [systematic name: (R,1R,2S)-rel--amino-2-carboxycyclopropaneacetic acid monohydrate], crystallizes with two organic molecules and two water molecules in the asymmetric unit. The space group is P2 1 and the organic molecules are enantiomers, thus this is an example of a 'false conglomerate' with two molecules of opposite handedness in the asymmetric unit (r.m.s. overlay fit = 0.056 Å for one molecule and its inverted partner). Each molecule exists as a zwitterion, with proton transfer from the amino acid carboxylic acid group to the amine group. In the crystal, the components are linked by N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds, generating (100) sheets. Conformationally restricted glutamate analogs are of interest due to their selective activation of different glutamate receptors, and the naturally occurring (+)-CCG-III is an inhibitor of glutamate uptake and the key geometrical parameters are discussed.

Structural commentary
The racemic title compound (Fig. 2) crystallizes as a 'false conglomerate' with two molecules of opposite handedness in the asymmetric unit. Each of molecules of 2-(2 0 -carboxycyclopropyl)glycine has a molecule of water hydrogen bonded to the glycine carboxylate group. It has been estimated that only 1% of organic compounds are false conglomerates (Bishop & Scudder, 2009).

Supramolecular features
In the crystal, the molecules are linked by N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds, forming sheets parallel to (100); Table 1 and Fig. 3.

Synthesis and crystallization
The racemic title compound was prepared according to the literature procedure (Wallock & Donaldson, 2004). A sample for X-ray diffraction analysis was recrystallized from water.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.